1. Field of the Invention
The present invention relates to an optical wave combining/splitting device that has a function to separate or demultiplex a wavelength-multiplexed optical signal into multiple optical signals in a specified selective wavelength band, and a function to combine or multiplex optical signals having various wavelengths.
2. Description of the Related Art
An example of a conventional optical wave combining/splitting device (that is, optical multiplexer/demultiplexer) is one disclosed in a document I (LEOS '95 Collection PD1.3). FIG. 6(A) shows the general configuration of the optical wave combining/splitting device disclosed in the document I. The conventional optical wave combining/splitting device comprises the first input/output port 11, a distributor 13 for distributing optical signal from the port 11, an arrayed waveguide diffraction grating wavelength splitter 15 connected to the distributor 13, and the second input/output port 17 comprised of multiple ports that is connected to the diffraction grating wavelength splitter 15. The diffraction grating wavelength splitter 15 further comprises the first star coupler 15a, the first arrayed waveguide 15b comprised of multiple waveguides having different optical path lengths, and the second star coupler 15c, which are sequentially positioned from the distributor 13 side. The distributor 13 can be implemented by a Y branching waveguide or a multi-mode waveguide.
The optical wave combining/splitting device executes optical signal separation operation when wavelength-multiplexed optical signals are input into the first input/output port 11. Among optical signals, each optical signal having a wavelength within the first selective wavelength band is output from the port 17a of the second input/output port 17, each optical signal having a wavelength within the second selective wavelength band is output from the port 17b, and each optical signal having a wavelength within the third selective wavelength band is output from the port 17c. The optical wave combining/splitting device executes wave combining operation when an optical signal in each selective wavelength band is input. The combined optical signal is output from the first input/output port 11.
In the optical wave combining/splitting device, optical signal which is input from the first input/output port 11 is broadened to 2.sup.j times by the distributor 13 and is input to the first star coupler 15a. Here "j" is a positive integer. FIG. 6 indicates the case when j=1. As a consequence, optical signals in a state were the band is substantially broadened and light intensity is flat are input into the first star coupler 15a, as shown in FIG. 6 (B). Therefore a selective wavelength band of the optical signal which is output from each port of the second input/output port 17 can be broadened, and also flatness of light intensity of each output optical signal having a wavelength within the respective selective wavelength band can be improved.
With the above described conventional optical wave combining/splitting device, however, light intensity (power) of each output optical signal having a wavelength within the same selective wavelength band drops because the distributor 13 broadens the optical signal which is input from the first input/output port 11. Consequentially, excess loss of optical signals occurs causing a problem.
For example, in order to broaden a selective wavelength band of each output port to double, while keeping light intensity of each output optical signal within the band substantially the same, a method for increasing another stage in a Y branching waveguide or a method for broadening mode width of the multi-mode waveguide to double is available, but regardless what method is used, 3 dB power loss occurs. If the selective wavelength band is broadened to 2.sup.j times, then j.times.3 dB of power loss occurs.
An object of the present invention is to provide an optical wave combining/splitting device (i.e., optical multiplexer/demultiplexer) which can implement expansion of a selective wavelength band and flattening of light intensity within the selective wavelength band as good as or better than prior art, and also has less power loss than prior art.